DataTable index over range

[DennisJensen95]

Hi,

When working with DataTables i ended up with index going over range. It is due to the boundaries of the indexing in object.rs.

failures:

---- dxf_parser::tests::test_parsing_dxf_with_ucs stdout ----
thread 'dxf_parser::tests::test_parsing_dxf_with_ucs' panicked at /Users/dennisjensen/.cargo/git/checkouts/dxf-rs-1994c28e8c467a91/b9613ed/src/object.rs:66:29:
index out of bounds: the len is 4 but the index is 4
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Resolution will be to change the boundaries.

[brettfo]

Fixed by #78.